Operating data centers is expensive with respect to floor space, cooling, energy demand needs, etc. Battery systems and battery storage cabinets employed in data centers are also expensive, are heavy, and have maintenance needs proportional to their physical size, energy storage, and production.
Data centers employ battery-powered backup systems known as un-interruptable power supplies (UPS). These systems may be integrated into a main electrical power source and automatically become operational when a power outage or power instability occurs. The UPS may supply the necessary power to keep the data center running until a standby generator starts or may permit an orderly shutdown of servers. It is not uncommon for a data center that experiences a complete power outage (i.e., a crash) and loses of all of its servers (without an orderly shutdown) to incur substantial monetary losses before re-starting.
UPS's that employ battery systems are supplied in large enclosed steel cabinets (and sometimes exposed racks). When fully populated with batteries, these cabinets can weigh four to five thousand pounds or more. Conventional cabinets may be the size of an old fashion pay telephone booth (e.g., about 7 and ½ feet high) and typically reside in the data center rooms. Typically these cabinets operate at about 480 volts DC and provide up to about 150 amp-hours (ah) of energy. For example, if a UPS is configured to consume full energy in 15 minute, the effective consumption rate is about 600 ah. When operated at four times rated consumption, these batteries are highly stressed in terms of a rapid rate of chemical reaction and production of heat.
FIG. 1 depicts an electrical block diagram of a conventional UPS system employing 3 battery backup sources. Some data centers use very large batteries (e.g., 300 to 650 lbs each) that are wired in series in battery groups 4a-4n known as strings and the resulting strings are additionally wired in parallel and coupled to a UPS bus bar 6 as indicated in FIG. 1 to provide for long run times (as current may flow out of the battery groups 4a-4n during discharge and into the groups 4a-4n during re-charge by the UPS as indicated by the arrows 8). The conventionally wired system 2 may be so large as to fill multiple rooms at considerable cost.
Conventional UPS systems employ a rated system voltage of 360 vdc to 480 vdc with some systems having rated voltages between these two values. Rated output voltages are derived from multiples of a fundamental unit cell voltage of 2 volts, 4 volts, 6 volts, 8 volts, 12 volts or 16 volts (e.g., 40, 12 volt batteries wired in series to produce 480 vdc). Since most data centers need large amounts of power, several strings may be wired in parallel to provide a constant 480 voltage at significantly higher current. For example, a small to medium size data center may have six battery cabinets at 480 vdc with 150 ah of capacity configured to be drawn down at the time rate of 15 minutes. Each cabinet may have 600 amps of deliverable power (over 15 minutes) times six, or 3600 amps of power times 480 vdc for a total output of 1.7 megawatts (mw) of power. Certain conventional large data centers may have battery backup power supplies of as much as 25 megawatts.
Unfortunately, each megawatt-hour of battery power may cost up to 125 thousand dollars or more and weigh as much as 30 thousand pounds, take up considerable space, and use significant power to recharge. As a rule, the bigger the data center system and/or the bigger the battery back up system, the more cost and energy demand expense and footprint size. Often these systems may be placed in a very expensive building that may be located in very expensive areas (e.g., Manhattan).
Another shortcoming relates to battery chemistry. Puekert's law expresses a measure of battery chemistry inefficiency at full power demand with respect to a battery's electrolyte-cell plate junction. At this junction, chemicals may be consumed at such a high rate that they cannot be replenished rapidly enough. As a result, energy output degrades and battery power (voltage) drops off quickly. A conventional fully charged battery cell has an output voltage per cell of 2.35 vdc. These cells may be arranged in series (e.g., 6 cells) to provide about times 14.1 volts for a 12 volt rated battery.
A fully charged 2.35 volt cell in a 12 volt battery (six cells) will safely provide current until the individual cell drops to 1.65 vdc or down to a total of 9.9 volts output per fully-configured battery, at which the battery is considered to be dead or out of power. In such circumstances, a battery cabinet or the end of each battery string is provided with a circuit breaker with a low voltage trip solenoid. When the string voltage drops under a certain value, the circuit breaker disconnects the batteries from the load and the power is cut off. A fully charged 480 volt (DC) rated battery cabinet/string having an operating initial voltage of 564 vdc is considered dead and will trip a low-voltage circuit breaker at about 396 vdc. Low-voltage circuit breaking is provided for a number of reasons: (1) the batteries do not have sufficient power and thus the UPS cannot produce sufficient power to run a data center; (2) a battery may be permanently damaged if its voltage drop is too low and may never be fully rechargeable; (3) thermal runaway may result; and (4) cell polarity reversal can occur with serious consequences.
Thermal runaway occurs when battery chemistry reacts at such a rapid pace that the battery heats to its melting point (with often dangerous out-gassing). At this point, even with the energy load disconnected, the battery is sufficiently damaged and the reaction will continue, causing more heat, up to and including the battery's self-ignition point where fire or a violent explosion may occur. As a result, there are some significant obstacles to power supply design.
Returning to Peukert's Law (i.e., Peukert's Equation; see below), several chemistry related problems may result when attempting to discharge a battery at a higher discharge rate than specified. In fact, by slightly discharging a battery above or near its rated discharge rate (e.g., a 150 ah battery may be rated to be discharged for about one hour for up to 15 minutes), chemical reactions may occur at a rate that passes a limit and the batteries' total capacity may be degraded by a factor of 1.3 to 1.4. This chemical limitation is related to “interface charge,” and from Peukert's Law, when a battery is charged or discharged, this action initially affects only the reacting chemicals which are at the interface (direct contact) between the electrodes and the electrolyte. With time, these chemicals at the interface, which may be called an interface charge, spread by diffusion throughout the volume of active material.
Peukert's Equation is a convenient way of characterizing cell behavior and of quantifying capacity offset in mathematical terms. Peukert's Equation is an empirical formula which approximates how the available capacity of a battery changes according to its rate of discharge. According to Peukert's Equation: C=InT, where “C” is the theoretical capacity of the battery expressed in amp-hours, “I” is the current, “T” is time, and “n” is the Peukert Number, a constant for the given battery. The equation shows that at higher currents, there is less available energy in a battery. The Peukert Number is directly related to the internal resistance of a battery. Higher currents translate to more losses and less available capacity.
The Peukert Number indicates how well a battery performs under continuous heavy currents. A value close to one indicates that the battery performs well; the higher the number, the more capacity is lost when the battery is discharged at high currents. The Peukert number of a battery is determined empirically. For Lead acid batteries, the number is typically between 1.3 and 1.4.